This invention relates to a method of making mercaptans. In particular, it relates to reacting hydrogen sulfide with an olefin in the presence of a heterogeneous acidic catalyst.
Mercaptans can be made by reacting olefins with hydrogen sulfide in the presence of a heterogeneous catalyst. Catalysts that have been used for this reaction include acidic clays such as acid washed montmorillonite clay (U.S. Pat. No. 2,950,324), promoted silicas (U.S. Pat. No. 2,951,875), zeolites (U.S. Pat. Nos. 4,102,931, and 5,453,544), doped acidic alumina (EU 0354460B1), and acidic ion exchange resins (CZECH 185469B1, U.S. Pat. No. 4,565,893, and U.S. Pat. No. 4,582,939).
The quality qf the catalyst is measured by the percent conversion of the olefin feed into a product and by the selectivity (i.e., the yield of the desired product). The best catalysts appear to be the acidic ion exchange resins. Optimized, they give conversions of over 95% and selectivities over 90% at hydrogen sulfide to olefin weight ratios of greater than or equal to 3 to 1 at 150 psig and 45 to 75.degree. C. with a throughput of 0.6 grams of olefin per mL of catalyst per hour. Unfortunately, acidic ion exchange resins begin to degrade at 100.degree. C. and substantially decompose at temperatures of 140.degree. C. Since the reaction of an olefin with hydrogen sulfide is highly exothermic, this imposes significant constraints on the use of this type of catalyst.